Humidity sensor

This is a continuation-in-part of application Ser. No. 196,902, filed Oct. 14, 1980, now abandoned.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to humidity or moisture sensing elements. The providing of suitable instruments for the measurement of relative humidity (RH) over wide ranges of RH (eg 10-90%) continues to be a challenging problem. While conventional humidity sensors have upper ohmic ranges of up to 10⁶ ohms a special electrode design and high impedance circuit in the present invention provide for routine measurement up to 10¹² ohms. A conventional humidity measuring system is limited in the ability to measure values above 10⁶ ohms because of noise pickup. Our new design presents very little antenna effect because of the small surface of the sensor and all the leads are isolated. The nature of the present humidity sensor allows for the miniaturizing of the sensor without sacrificing lifetime and stability. Because the impedance is high, the electrical currents in the sensor are minute and the I² R heating effect is very low. The sensor can thus be reduced in size. The sensor might be described as one having an extended impedance range in that while the impedance at high humidities (i.e. 90% RH) is in the order of 10³ ohms (this being similar to existing RH sensors), the impedance at low humidities (i.e. 10% RH) is in the order of 10¹² ohms with a 10⁶ impedance at about 35% RH. Thus the improved sensor extends the range into the higher impedances moving up from prior devices at about 10⁶ to about 10¹² ohms. Thus when referring to a high impedance sensing element herein we mean that at the low range of relative humidity (under very dry conditions), the resistance from conductor 11 to conductor 12 through the iron oxide is in the order of 10¹² ohms.

In the prior art it has been known to use iron oxide in a humidity sensitive element as has been described in the article "Electrical properties of iron oxide polyethylene glycol humidity sensitive elements" by Nicholas, Pitkanen, Lavine, Zook and Hagen, May 1976, Journal of Applied Physics, Vol. 47, No. 5 Pages 2191-2199. That disclosure, however, was directed to a much lower impedance device, the polyethylene glycol being combined with the Fe₂ O₃ for the specific purpose of reducing the impedance.

When a high impedance device (10¹² ohms) of the nature of the present invention is desired there are special problems to be overcome, one of which problems is that of leakage currents. The novel design of the present detector is specifically directed to overcome this problem. Thus the described structure is easy to make and yet easy to control stray leakage paths. The low impedance guarded structure reduces pickup by providing shielding. The compact structure provides low electrical pickup in that the small geometry results in small capacity to radiating electric fields.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of one embodiment of a high impedance humidity sensor according to the invention.

FIG. 2 is a cross sectional view of the sensor of FIG. 1 as well as of further sensor supporting structure.

FIG. 3 is a modification of FIG. 1.

FIG. 4 is a graph of impedance vs. RH of the humidity sensor.

FIG. 5 is an electrical schematic presentation of the high impedance humidity sensor and the related circuit.

DESCRIPTION

A high impedance relative humidity sensing element is provided by applying onto an insulating substrate a conductive film such as gold, the perimeter of the first film being completely encompassed by but spaced from a second conductive film on the non-conducting substrate. A humidity responsive layer of iron oxide is applied over the conductive film layers and the spacing, to bridge the spacing in a continuous manner with the humidity responsive layer. Electrical terminals are connected, respectively, to the two conductive film layers.

Referring now to FIGS. 1 and 2 there is shown an insulating substrate 10, such as non-conducting high density high purity ceramic or glass. Examples are AlSiMag, alumina, sapphire and quartz. The substrate has applied on the upper surface thereof an inner conductive film 11, such as evaporated gold, and an outer conductive film 12, also evaporated gold. A spacing belt 13 of about 0.003" width, for example, may be used between the inner and encompassing films. In one successful embodiment the substrate used was 3 M 614 AlSiMag high-density high-purity 0.025" thick, cut into 1" squares. Two small holes 14 and 15 may be drilled through the substrate for bringing in the lead wires 16 and 17 from the back side if desired. One entire cleaned surface was coated with evaporated gold except for the 0.003" spacing belt. The gold may be applied by silk screening if desired. A firing process follows. The four edges were masked to about 1/16" onto the gold surface 12 as well as a circular area 1/16" in center on surface 11 and then the surface was uniformly coated with the humidity sensitive iron oxide. The iron oxide suspended in a dilute aqueous solution of polyvinyl alcohol was applied with an airbrush in one instance, until the applied oxide film was sufficiently opaque to obscure the gold of the electrode layers 11 and 12. This may require heaters to dry the oxide between sequentially applied layers. The iron oxide was then sintered at about 1080°-1100° C. for about 13-15 minutes to complete the sensor. During the sintering process the iron oxide powder interlocks to form porous balls and also attaches to the electrodes. Unsintered iron oxide sensors may also be used, if desired.

While the invention has been described in terms of the humidity sensor layer or film being of iron oxide (Fe₂ O₃) as the preferred embodiment, the humidity sensitive layer material more generally may be selected from a hygroscopic metal oxide, a hygroscopic polymer (doped or untreated) or an inorganic salt. It is known that these materials, when in proper form, exhibit the high resistance of electrical insulators. It is known that certain of the oxides are hygroscopic in nature which affects their insulating properties such that when exposed to an atmosphere containing a quantity of water vapor, the value of electrical resistance changes as a function of the water vapor in the atmosphere. Desirable properties including humidity dependent resistances and a high degree of chemical stability are especially prevalent in the oxides of the metals of Group VIII, Period 4 of the Periodic Table of the Elements. Thus while others may be used, the oxides of iron, nickel and cobalt are preferred, in accordance with the present invention. As to the doped polymer an example of a suitable material is polyvinyl alcohol with lithium chloride. Suitable untreated polymers include polyethylene fluoride and polyimides. Suitable inorganic salts include lithium fluoride, lead iodide, cerium titanate and sodium hydrogen phosphate.

In the embodiment of FIGS. 1 and 2 the outer electrode 12 is shown as being concentric around the inner electrode 11 and the spacing 13 between the electrodes appears as a ring shape. While it is necessary that the spacing ring 13 close on itself and be continuous, it is not necessary that it describe a circle with a fixed radius. The spacing ring 13 may have a generally rectangular design or may be serpentine to increase the path length as shown in FIG. 3. In each case the iron oxide is applied over the spacing ring and the inner and outer conductive electrodes to bridge the spacing ring with iron oxide in a continuous manner. If the lead 16 is brought out through the back side as shown in FIGS. 1 and 2 then it is desirable to have a guard ring 18 on the back side of the substrate.

The technique of guarding is utilized in the present sensor structure. When designing with high impedance sensors, stray leakage paths caused by surface contamination result in errors in measurements. To avoid the requirement for maintaining clean uncontaminated surfaces in the operating environment it is often desirable to provide a terminating path for the leakage currents. One technique is called guarding. A guard ring provides a conductor shield around the sensor signal sense point. This guard ring is maintained at the signal electrical potential by means of electronic circuitry. Since there is no electric potential between the guard ring and the signal sense point, there is no stray conducted leakage signal to cause measurement error.

Referring now to the schematic circuit of FIG. 5, the relative humidity sensor of FIGS. 1 and 2 is shown as resistive element 30. Element 30, having the generally circular geometry, is effectively bounded at the outer edge of the outer conductive film 12 by a shield 31, here shown in dotted form. The shield is connected to film 12 and conductor 17 in FIG. 5, with conductor 17 being connected to one terminal of an AC generator 32, the other terminal of which at ground. The generator may be, for example, a 1 Hz or a 10 Hz square wave signal of about 1 volt peak-to-peak. The signal need not be square wave. The inner conductive film 11 of the sensor is connected by conductor 16 to a junction 33 and to the positive signal input of a FET (low bias current) OPAMP 34 operating as a feedback buffer. Junction 33 is also connected to a high megohm resistor 35, the other terminal of which is at ground. A guard ring 36 surrounds junction 33, the guard ring being directly connected to the other (negative) input of OPAMP 34. With the feedback circuit around OPAMP 34, there is a zero potential difference between the two inputs whereby guard ring 36 is maintained at the same potential as the signal input to prevent leakage currents. The output of OPAMP 34 is connected to amplifier 37 which controls an indicator such as meter 38.

A low impedance guarding signal can easily be obtained by the use of a "unity gain" feedback amplifier as OPAMP 34. By adding feedback from the output to the inverting (negative) input we have a low impedance signal that tracks or equals the input signal. This low impedance point provides the guarding voltage.